Dissolution of metal with acidified hydrogen peroxide containing dibasic acid



United States Patent 3,341,384 DISSOLUTION OF METAL WITH ACIDIFIED HYDROGEN PEROXIDE CONTAINING DI- BASIC ACID Carmelo L. Alderuccio, Camillus, and Harold F. Jones,

Marcellus, N.Y., assignors to Allied Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed May 4, 1964, Ser. No. 364,798 11 Claims. (Cl. 156-18) Thisinvention relates to dissolution of metal With hydrogen peroxide, and more particularly to acid-hydrogen peroxide solutions as an etchant for copper as in the manufacture of printed circuits.

As an etchant for copper, aqueous hydrogen peroxide is very attractive because of its relatively low cost and ability to recover copper electrolytically from the spent peroxide etchant solutions. However, the utilization of hydrogen peroxide for a constructive purpose in metal etching is far from a simple matter primarily because of the highly depreciating and exhaustive eifect on the etchant solution caused by the etched metal in the form of ions which rapidly increase in concentration in the etchant. In co-pending application Ser. No. 334,572, filed Dec. 30, 1963, now US. Patent No. 3,293,093, there are described especially effective metal etchant solutions based on acidified-hydrogen peroxide solutions which contain phenacetin, sulfathiazole and/ or silver nitrate and also hydrogen peroxide concentrates containing phenacetin from which acidified 4-l2% hydrogen peroxide etchant solutions can be readily produced by addition of acid and water. With respect to said concentrates it was diound that those containing the larger amounts of phenacetin tended to lose some of the phenacetin by crystallization when stored for long periods of time at substantially reduced temperatures as during the winter months. The crystallized phenacetin is diflicult to later redissolve in the concentrate or resulting etchant solution. Thus, analysis of the stored material and adjustment by addition of phenacetin to replace the lost material became a factor in the use of such concentrates.

An object of the present invention is to provide new and improved hydrogen peroxide concentrates containing phenacetin and suitable for conversion to hydrogen peroxide solutions for dissolution of metal.

Another object of the invention is to provide new and improved hydrogen peroxide concentrates containing phenacetin at a concentration below which substantial amounts crystallize at reduced temperatures and yet suitable for conversion by addition of acid and water to new and improved etchant solutions having high etch capacity and fast etch rate.

Another object is to provide an additive for use in combination with phenacetin to obtain acidified hydrogen peroxide solutions for etching of metal in a highly eflicient manner and at fast etch rates.

A further object is to provide new and improved etchants based on hydrogen peroxide.

A still further object is to provide new and improved hydrogen peroxide etchant solutions for etching of copper for a constructive purpose as in the manufacture of printed circuits.

Other objects and advantages will be evident from the following description of the invention.

It has been found in accordance with the present invention that saturated dibasic acids of 4 to 12 carbon atoms may be substituted in part for phenacetin without any substantial reduction in the high etching efficiency demonstrated by the acidified hydrogen peroxide etchants containing substantially greater amounts of phenacetin. As an additive in the acidified peroxide metal etchants the dibasic acids by themselves were found much less effective than phenacetin, particularly in spray etching procedures, and no explanation can be given with certainty as to improved effectiveness obtained when the dibasic acids and phenacetin are combined. The new etchants provided by the invention also have the particular advantage of being preparable from the hydrogen peroxide concentrates containing the dibasic acid and a correspondingly less amount of phenacetin below the level of substantial crystallization and loss from the concentrate at reduced temperatures, said concentrated hydrogen peroxide solutions containing 20-70% by weight hydrogen peroxide, between about 200-2000 parts per million phenacetin and between about 200 to 10,000 parts per million of a saturated dibasic acid of 4 to 12 carbon atoms, preferably 4 to 9 carbon atoms. It has also been found in accordance with this invention that improved etching capacity is obtained on combining the dibasic acids with sul-fathiazole or silver ions and further that tertiary additive systems containing phenacetin, silver ions, and the dibasic acids are useful in providing acidified hydrogen peroxide etchants of further improved capacity over the highly effective etchants containing the same total amounts of a binary additive system made up of phenacetin and silver ions.

The dibasic acids which may be employed in the invention in the etching of metal are the saturated acids of 4-12 carbon atoms, inclusive. Included among such dibasic acids are adipic acid, succinic acid, azelaic acid, sebacic acid, pimelic acid, suberic acid, glutaric acid and malic acid. The substituted dibasic acids are also useful where the substituent is hydroxy and/or carboxyl, provided at least one of the carbon atoms adjacent the terminal dibasic acid carboxyl group is free of said hydroxy and carboxyl substituents. For example, malic acid and citric acid may also be employed while tartaric acid does not :give satisfactory results. Salts yielding the dibasic acids in the acidified etchant may also be employed. The preferred dibasic acids are the unsubstituted acids of 4 to 9 carbon atoms and include adipic, succinic, azelaic, and pimelic acid. The more preferred acid is adipic acid. In the acidified hydrogen peroxide etchant solutions only a small amount of dibasic acid is required to have the desired catalytic effect. As little as about 40 parts per million of the dibasic acid may be combined with phenacetin, sulfathiazole, and/or silver ions to provide an etchant of high capacity. Increasing the amount of dibasic acid will further benefit etch rate and capacity. An amount of dibasic acid in excess of 3,000 parts per million offers no added advantage and is undesirable from a process and economic standpoint. Preferably, the amount of dibasic acid is between about 75 to 500 parts per million. At least about 30 parts per million of phenacetin or sulfathiazole is used in combination with the dibasic acids with preferably between about 75 to 500 parts per million being employed. The

amount of free silver ions combined with the dibasic acids may be as little as 10 parts per million, preferably between about 50-500 parts per million. Preferably, the combined additive systems containing the dibasic acids will total about 150-1500 parts per million of additive. A particularly preferred solution has incorporated therein a tertiary additive system containing between about 75-500 parts per million dibasic acid, to 300 parts per million phenacetin and about 50 to 500 parts per million free silver 1011.

The concentrates containing 20-70% by weight hydrogen peroxide will contain between about 200-2000 parts per million phenacetin, preferably 400-1000 parts per million, and between 200-10,000 parts per million dibasic acid, preferably between a out 600-2500 parts per million. The more preferred concentrates will also contain between 200-5000 parts per million free silver ions, preferably between about 500-2500 parts per million free silver ion. The silver ions are preferably furnished by addition of silver nitrate in an amount between about 300 7000 parts per million, preferably between about 750- 3500 parts per million. The preferred concentrates contain between about 30-60% by weight hydrogen peroxide. The etchants containing phenacetin may be readily prepared by simple addition of acid and water to the peroxide concentrates. If desired, other additives such as sulfathiazole may be incorporated by simple mixing. The peroxide concentrates provided by the invention may be easily and safely shipped and have the further advantage of being storable for extended periods of time at varying temperature conditions including winter temperatures Without substantial loss of the phenacetin by crystallization.

In preparation of the etchants it is important that the solution contain less than 2 parts per million total free chloride and bromide ions, preferably less than 1 part per million. When phenacetin is the only additive combined with the dibasic acid special consideration is required to obtain an etchant of desired low chloride and bromide concentration. Deionized water may be used in make-up of such etchants to assure less than 2 parts per million of chloride and bromide ions. If desired, ordinary water may be employed if accompanied by addition of suitable material capable of removing chloride and bromide ions. In a preferred embodiment a small amount of a watersoluble silver salt, preferably silver nitrate, is added to effect the removal of chloride and bromide ion and furnish free silver ions to catalyze the etching. The precipitated silver halide matter in the acid-peroxide-phenacetin solution does not interfere with the etching process. Solutions having incorporated herein a tertiary additive system containing the dibasic acid, phenacetin and free silver ions are the more preferred and exhibit exceptionally fast etch rates and high capacity significantly greater than obtained when the additives are used alone or in binary combinations. For example, the indicated tertiary system surprisingly improves etch rate and capacity over a system containing the same total amount of only phenacetin and silver ions. When sulfathiazole is employed it has been found that special consideration may be dispensed with. While the explanation for this result is uncertain it is evident that sulfathiazole functions not only to increase the etching capacity of peroxide solutions but also to negate the adverse repressive effect of the chloride and bromide ion concentration on etch rate and capacity. Hence, the addition of sulfathiazole has the advantage of permitting use of ordinary tap water in preparation of the etchant without special treatment as required when phenacetin is used alone. When employing sulfathiazole either in combination or alone in etchants prepared from ordinary tap water between about 150 to 250 parts per million are usually required to negate the adverse effects of the free chloride and bromide ions in the water. The capacity of sulfathiazole in overcoming the adverse effect of chloride and bromide ion is apparently not unlimited. Solutions containing the higher concentrations of these ions, say above about 30 parts per million, will require additional treatment to negate this effect of such ions, e.g., deionization or addition of a soluble silver salt.

In the dissolution of metal by the invention the hydrogen peroxide concentration may vary over a fairly wide range. Etching of copper metal is desirably carried out in acidified solutions having a hydrogen peroxide concentration between about 2-12%. At solution cncentrations less than about 2% by Weight etch rates are impractically low and etching unsatisfactory. At concentrations about above 12% by weight it has been found that copper metal may be etched but the dissolution of the etched copper ions in the etchant causes decomposition of the peroxide with the result that etching at such high concentrations is less economical. The best results are obtained in solutions having a peroxide concentration between about 2l0%. During the etching process hydrogen peroxide is consumed as more and more amounts of copper are treated. In order to be practical it is necessary that a single etchant dissolve a substantial amount of copper metal before the solution becomes exhausted to the extent that a particular workpiece cannot be etched within a reasonable time, e.g., 12 hours. The hydrogen peroxide solutions employed in the invention must therefore have an initial hydrogen peroxide concentration of at least about 4% in order to dissolve sufficient metal to be practical from an economic standpoint. Desirably, the etchant solution has initially a hydrogen peroxide concentration within the range of about 5-10% by weight. The hydrogen peroxide solutions having the indicated initial hydrogen peroxide concentrations are useful in etching a single large copper I piece or a series of workpieces containing limited amounts of copper. The etchant is capable of operating eifectively at good etch rates after partial exhaustion and at high dissolved copper concentrations equivalent to at least 10 ounces of copper per gallon and even substantially higher.

The acid concentration may also vary considerably. In copper etching it is desirable that the etchant solution have a hydrogen ion concentration from about 0.45 t about 5.5 grams per liter, preferably between about 0.65- 4.5 grams per liter. Below a hydrogen ion concentration of about 0.45 gram per liter the etch rate is slow and peroxide decomposition high, particularly after partial exhaustion of the peroxide bath. The desired upper limit of the hydrogen ion concentration may depend on several factors including the particular acid employed. A hydrogen ion concentration above about 5.5 grams per liter is generally less economical and tends to slow down rather than increase the etch rate. Inorganic acids and even the stronger organic acids such as acetic acid may be used to supply the hydrogen ion concentration in the etchant solution. Examples of the acids which are the more suitable for supplying the hydrogen ion concentration include sul furic acid, nitric acid, and fiuoboric acid. Nitric acid has been surprisingly found to be useful in etching copper without release of any substantial amounts of toxic nitrogen oxide vapors which would normally be expected in such a process. The use of any significant amount of hydrochloric or hydrobrornic acid is of course desirably avoided because of the introduction of large amounts of chloride and bromide ions which have a retarding eifect on etching and must be negated, removed, or otherwise provided for in order to obtain a practical etching. The acid preferably employed in peroxide etching of copper is sulfuric acid. The amount of sulfuric acid in the hydrogen peroxide etchant is between about 2-23% by weight, preferably between about 320% by weight. Sulfuric acid concentrations above about 23% are less desirable as tendency to result in less uniform etching. This effect is apparently caused by the formation of a protective coating on substantial portions of the exposed copper surface which is thereby made resistant to etching. The influence of the acid concentration on the copper etch rate has been found interesting and worthy of note. When the acidified hydrogen peroxide etchant solution contains only minor amounts of dissolved copper the effect of acid concentration on etch rate is negligible and the full range of hydrogen ion concentrations between about 0.45 to 5.5 grams per liter results in little variance in etch rate. A the peroxide bath becomes more exhausted and dissolved copper concentration increases, the influence of the acid concentration increases markedly. At the higher dissolved copper concentrations both the lower and higher acid concentrations result in longer etch times. An optimum etch rate has been found to be reached at an intermediate hydrogen ion concentration between about 0.9 to 1.4 grams per liter (about 46% by weight sulfuric acid). In the etching of copper with the acid-hydrogen peroxide system of the invention both the hydrogen peroxide and acid are theoretically consumed at a rate equivalent to a mol ratio of hydrogen peroxide. Thus, according to the etching reaction one mol of sulfuric acid is consumed for each mol of peroxide and the acid concentration slowly decreases as the dissolved copper concentration increases. As the acid concentration does not have a substantial elfect on etch rate at low dissolved copper concentrations it will be noted that the hydrogen peroxide etchant may contain initially a high hydrogen ion concentration with relatively little sacrifice of etch rate after partial exhaustion and increase of the dissolved copper concentration. In situations where it is desired to optimize etch rates and employ lower acid concentrations the etchant solution may be advantageously made up to contain initially a low or intermediate hydrogen ion concentration, of the order of about 0.45-3.4 grams per liter (about 2-15 by weight sulfuric acid), preferably between about 1.l2.6 grams per liter (about 512% by weight sulfuric acid). Then, as the etchant is consumed causing reduction of the hydrogen ion concentration additional acid is added to regulate the hydrogen ion concentration within the optimum range of about 0.9-1.4 grams per liter (about 4-6% by Weight sulfuric acid). Addition of the acid may take place either continuously or intermittently and either immediately after the start of the etching or after significant exhaustion of the ecthant solution. When the initial hydrogen ion concentration is low, say of the order of about 0.451.1 grams per liter (and 25% by weight sulfuric acid), the addition of the acid preferably takes place substantially immediately after etching commences and is desirably more or less continuous until the hydrogen ion concentration is increased to well within the range of about 0.9-1.4 grams per liter. When the initial hydrogen ion concentration is greater than about 1.1 grams per liter the addition of acid to maintain the optimum concentration preferably takes place from time to time and after the etchant solution has been exhausted to the extent that the hydrogen ion concentration is below about 1.1 grams per liter, usually just after the 0.9 gram per liter.

In the etchant solution the ratio of hydrogen per-oxide to acid is less important than the concentration of the acid. As the chemical reaction or mechanism by which copper is etched consumes one mol of peroxide and 2 mols of acid hydrogen in a mol ratio of l to 2 is indicated, i.e., a H O /H+ ratio of 1 to 2. Peroxide to hydrogen ion mol ratios less than 1 to 2 are therefore generally unnecessary and may tend to slow the etch rate, particularly at the higher reagent concentrations. In practice, the amount of hydrogen peroxide actually consumed seldom will exceed about 75% so that the inclusion of just slightly more than about 1.5 mols of hydrogen ion per mol of peroxide will be adequate to supply sufiicient acid for complete utilization of the particular etchant solution. As some peroxide is also not utilized because of decomposition the etchants made up to include sufficient acid for complete utilization without addition of more acid preferably have a hydrogen ion mol ratio of not less than about 1.0 to 1.6, and desirably in the rangeof about 1.0:1.6 to 1.0:1.0.

When acid is to be later added and the etchant solution contains initially a low or intermediate acid concentration the mol ratio of peroxide to acid hydrogen may of course be initially somewhat greater, preferably between about 1.0:0.2 to 1.0:1.0. As hydrogen peroxide is consumed and more acid added the mol ratio of peroxide to acid will b reduced and eventually become similar to the mol ratios preferably employed in the solutions made up to contain the complete acid requirement. Again, because peroxide utilization seldom exceeds 75%, it is desirable from a practical viewpoint not to add an amount of acid sufiicient to reduce the mol ratio of peroxide to acid hydrogen below about 1.0 to 1.6.

Temperature of the acidified-hydrogen peroxide solution is another important factor in etching copper. As a practical matter copper metal is not etched at room temperatures or below. The nature of the attack of the acid hydrogen peroxide solution on copper at such temperatures is more of a polishing, oxidizing or brightening etconcentration is reduced below about feet. In order to efliciently etch copper the hydrogen peroxide solution must have a temperature of at least about 40 C. at time of contact with the metal. Solution temperature has a strong effect on etch rates and increasing the temperature to a preferred range between about 50-62 C. will substantially increase the rate of etching to a level significantly greater than heretofore realized with ammonium persulfate etchants at recommended optimum temperatures. At hydrogen peroxide solution temperatures above about 65 C. little further increase in etch rate is realized and such temperatures have been found undesirable as resulting in an impractically high rate of peroxide decomposition. As is the case with acid concentration the influence of temperature on etch rate has been found to be greatest after partial exhaustion of the etchant and increase of the dissolved copper concentration. If desired, etching may be commenced at the lower temperatures, for example, between about 40 C. to 55 C., and temperature of the solution then gradually increased up to a higher temperature of approximately 55-62 C. as the solution is further exhausted. Increasing the temperature of the etchant solution is aided by the etching reaction itself which is moderately exothermic. Increasing the temperature of the etchant may be used to advantage to regulate etch rates at a more or less constant value when a number of pieces are to be etched in the same solution such as, for example, when employing automatic systems used in the manufacture of printed circuits.

A particular feature of the invention is that it may be utilized in that phase of the manufacture of printed circuit boards involving the etching of copper clad laminates to obtain the conductive pattern. Such etching is wellknown and need not be described herein in great detail. The laminates from which the circuit boards are produced are usually composed of a thin copper sheet laminated to a base sheet of electrically insulating material which is typically a polymeric vinyl chloride plastic. Other electrically insulating materials to which. the copper may be laminated include ceramics, glass, and the phenolic, epoxy, melamine, silicone and fluorocarbon resins. Thickness of the copper sheet in such laminates may vary considerably, say from about A mil up to about 10 mils or more, usually between about /2 mil to 5 mils. It is conventional to gauge the thickness and amount of copper in such laminates in terms of ounces of copper per square foot. For example, a laminate having a copper sheet of about 2.7 mils thickness is commonly designated as a 2 ounce copper board. The conductive pattern desired on the board is outlined by a masking or resist material which of course must be highly resistant to attack by the chemical agents employed in the etching step. Several types of resist materials are well-known and available commercially. Among such conventional materials found most suited for use with the peroxide etchant of the invention are Advance Plating Resist R-9l843 supplied by Advance Process Supply Company, Meaker Etch No. 200 supplied by Meaker Company, Sel-Rex Corporation, Candoc SS1105, Toluidine Red supplied by Cudner & OConnor Company, Candoc $31139, Perma Peacock Blue supplied by Cudner & OConnor Company, KP Acid Resist No. 250-15 supplied by Kresslik Products Company, Economy White A 11632 supplied by Union Ink Company, Printed Circuit Resist Black supplied by Union Ink Company, and Kodak Photo Resist supplied by Eastman Kodak Company. A particular feature of the invention is that it is suitable for both immersion and spray etching. Agitation of the bath or workpiece is desirable as conventional in immersion etching procedures. In immersion etching a bath containing initially about 8% by weight hydrogen peroxide is preferred as a practical matter in obtaining the lower cost per weight unit of copper etched. In spray etching a solution containing initially about 16% by weight hydrogen peroxide is preferred. The peroxide solutions prepared from ordinary water to which a soluble silver salt has been added to remove chloride and bromide ion may of course be employed in both immersion and spray etching. In spray etching procedures the etchant should include either phenacetin or sulfathiazole, preferably phenacetin in combination with a material furnishing silver ions. The amount of silver ion required to realize optimum results is apparently somewhat greater in spray etching. The amount of silver ion added to spray etching solutions therefore is preferably between at least about 75 up to about 500 parts per million, desirably in the range of about 100300 parts per million. In practice, etch contact time of the workpiece with the etchant depends on several factors including particularly thickness or amount of copper to be etched, concentration or extent of exhaustion of the peroxide and acid in the bath, temperature, degree and method of agitation. A thin copper sheet may be etched at the higher permissible temperatures in a freshly made solution of high peroxide concentration in as little as about A minute. Etching of successive boards requires longer time although a number of copper laminates of conventional copper weight may 4 be etched with the peroxide solution over a fairly constant time period which is also a desirable feature. The longer contact times are mostly a matter of economics and .capability of the highly exhausted bath to complete the etching within a reasonable period. Contact for about 60 minutes generally represents the practical upper limit for the hydrogen peroxide etchant of the invention. Contact times between about /2 to 50 minutes are preferably employed in etching a series of copper laminates of /2 to 5 mil thickness in the manufacture of printed circuits. Etch rates in manufacture of printed circuits may of course also be controlled by regulation of bath temperature and acid concentration as found possible With the peroxide etchant of the invention. Thus, the peroxide solution temperature may be slowly increased to provide a more constant etch time in treating a series of copper laminates. Acid concentration may be minimized and etch rates maximized by employing a solution having initially a low or intermedate acid concentration and, after partial exhaustion, adding more acid to regulate the hydrogen ion concentration within the optimum range of about 0.9-1.4 grams per liter. In the etching of printed circuit boards an important consideration is undercut which measures the degree to which the etchant acts horizontally beneath the resist material compared to the desired action vertically toward the underlying plastic base. Undercut is generally defined as a ratio of copper sheet thickness to the amount of horizontal attack under the resist material. A ratio better than 1 to l is desired for satisfactory results. The hydrogen peroxide etchant of the invention has been found highly satisfactory in this respect in demonstrating an undercut ratio of about 2.

The following examples in which parts and percentages are by weight unless otherwise noted demonstrate the practice and advantages of the present invention.

The copper clad laminates employed in the following examples were supplied by General Electric Company under trademark Textolite (No. 11571). In Examples ll2 the copper laminates were cut into board specimens having dimensions of 2%, x 4 x inch. Each specimen had about 0.14 total ounces of 2.7 mil thick copper (2 ounces per square foot) laminated to a plastic base. In Examples 1-12 etching was carried out by immersion of the specimens in 500 gram solutions contained in 500 ml. tall beakers with a water bath used for control of this etchant bath temperature. Etching of the specimens having 0.14 ounce of copper in 500 grams of solution may be reported in terms of etch time at known ounces of copper dissolved per gallon of etchant in which terms the results in the examples are expressed. The specimen to be etched was attached at one of its ends to a reciprocating mechanism adapted to agitate the specimen up and down through a displacement distance of about /2 inch at a rate of about 50-60 strokes per minute. The 500 ml. beakers were charged with 500 grams of acidified hydrogen peroxide solution and etching commenced by introducing the specimen into the resulting peroxide bath While commencing agitation of the specimen. Etch time was determined with a stopwatch and etch rate calculated by weighing the specimen before immersion and after withdrawal from the bath. Etch time is expressed in terms of the time in minutes required to remove all of the exposed copper from the test specimen.

Examples 1-8 Employing the immersion procedure outlined above eight different etchant solutions were evaluated in etching a series of the copper clad specimens in each solution. All etchant Baths AH, inclusive, contained 8% by weight hydrogen peroxide and 17.3% by weight sulfuric acid such that the mol ratio of peroxide to acid was about 1 to 0.75. The peroxide etchants were prepared with deionized water and contained only about 0.2 parts per million total free chloride and bromide ions. Bath A contained as additive only adipic acid in an amount of 400 parts per million, while Bath B contained only 400 parts per million phenacetin. Bath C contained a combination of 240 parts per million adipic acid and 128 parts per million phenacetin. Bath D was the same as Bath C except there was added about 267 parts per million silver nitrate. Bath E contained only 400 parts per million sulfathiazole while Bath F contained a combination of 240 parts per million adipic acid and 160 parts per million sulfathiazole. Bath G contained as additive only silver nitrate in an amount of 267 parts per million. Bath H contained a combination of 180 parts per million adipic acid and 90 parts per million silver nitrate. The hydrogen peroxide etchant baths were regulated during etching at a temperature of about C. Results summarizing Examples l 8 are given in Table 1.

TABLE 1 Etch Rate of Bath, Minutes Bath A Bath B Bath 0 Bath D Bath E Bath F Bath G Bath H Concentration, Ounces Copper Dissolved Per Acid Pe oxide,

Gallon of Acid Peroxide, 240 p.p.m. Acid Peroxide, Acid Peroxide, Etchant Acid Peroxide, Acid Peroxide, 240 p.p.m. Adipic Acid, Acid Peroxide, 240 p.p.m. Acid Peroxide, 180 p.p.m.

400 p.p.m. 400 p.p.m. Adipie Acid, 267 p.p.m. 400 p.p.m. Adipic Acid, 267 p.p.m. Adipic Acid, Adipic Acid Phenacetin 128 p.p.m. Silver Nitrate, Sulfathiazole 160 p.p.m. Silver Nitrate p.p.m.

Phenacetin 128 p.p.m. Sulfathiazole Silver Nitrate Phenacetin 9 10 Table 1 shows generally good etch rates for the acidthe combination of adipic acid and phenacetin in imified hydrogen peroxide etchant baths A-H, inclusive, mersion procedures with acidified peroxide etchants prewhich contain less than 2 parts per million total free pared with ordinary tap water. chloride and bromide ions. The hydrogen peroxide Bath A containing adipic acid is less effective both as to etch 5 Examples 13*24 rate and capacity than Bath B which contains the same amount of phenacetin.

Bath C containing the combination of adipic acid Additional dibasic acids were evaluated by an immersion procedure similar to that outlined above except that after etch time exceeded 4 minutes (at about 6 and phenacetin in a total amount of less than 40 0 tot ounces dissolved copper), two boards were etched at a P i P mllllOIl PP T Y the same 1113i} P time by placing the board specimens back to back in pacity as iBath B which contains 400 parts p r Inllllon the etchant. Each of the solutions tested (Baths M-X, Phenacetm- Companson of f B therefore inclusive) contained 8% by weight hydrogen peroxide illustrates the improved effectiveness of adlpic acid when and 175% by Weight lf i acid Each solution was combined with phenacetin. Bath D shows that the addiprepared f ordinary tap water and had incorporated t1011 of 267 P P mlnlon sllvef filtrate to Bath C 15 therein 267 parts per million silver nitrate. The amount results in further improvement and especially fast etch f dibasic acid employed was 2000 parts per million rates and 1 1igh capacity- A comparison of etch rates while the solutions containing phenacetin contained 100 and capacities of Bat-l1 A, E and F shows that the comrts per million of this additive. During etching each blnatloll adlplc acld and Sulfathlazole 1n Bath F of the solutions was regulated at a temperature of 60 results in improvement over Baths A and E in which Q Results summarized below in Table 3 Show the etch these f f are employed separately in the same rate in minutes for each solution with approximately amount slmllarly, a Compansoll of Baths G and 9.5 ounces of copper dissolved therein per gallon.

H shows that the combination of adipic acid and silver nitrate results in improvement in capacity over that ob- TABLE 3 tamed separately wth t e e a d1 ive Bath Additives Etch Rate Examples 9-12 Four additional peroxide Baths I-L were prepared BathM (silver Nitrate only) similar to those in Examples 1-8 except that ordinary BathN Phenecetin 7.0 tap water was employed. Each bath contained about 8% Bath 0 Mime Aqid 6' 5 hydrogen peroxide and 17.3% sulfuric acid. Bath I con- Bath P Adlplc Aeld p s P n cet n 5.0 tained no additive and about 5 l0 parts per million BathQ succmc Acid M total free chloride and bromide 1011. Bath J contained BathR 8110011110 A0111 p cet n 5.6 about 490 parts per million adipic acid and 267 parts Baths Azelaie Acid 4'8 per m1ll1on s1lver nitrate. Bath K contained about 400 BathT Alelalc Acid P1118 Phenacetin- 4.3 parts per million phenacetin and had incorporated there- Bath U Sebacic Acid 5 in 26 7 parts per million silver nitrate. Bath L was pre- Bath Sebacic Acid P1115 Phenacetin pared by addition of 128 parts per million phenacetin, BathW Pirnelie Acid 6.4 240 parts per million adipic acid and 267 parts per BathX Plmelic Acid P1115 Phenacefln million silver nitrate. Each bath was regulated during etching at a temperature of about C. Results sum- In Table 3 Baths M-P, inclusive, are included for marizing Examples 9-12 are given in Table 2. comparative purposes. The remaining Baths Q-X, in-

TABLE 2 Etch Rate, Minutes Bath I Bath J Bath K Bath L Concentration,

Ounces Copper Dissolved Per Gallon Peroxide, 240

of etchant Peroxide, 400 Peroxide 400 p.p.m.Adipic Peroxide, ppm. Adiplc p.p.m. P en- Acid, 128

No. Acid, 267 acetin, 267 p.p.m. Phen- Additive p.p.m. Silver p.p.m. Silver acetin, 267 Nitrate Nitrate ppm. Silver Nitrate Initial Rate 7 O 1.2 1.0 1.4 2 7 1 1.4 1.5 1.6 4-- 7 5 2. 7 2. 5 2.4 9 4 5.2 a 7 as 5 0 9.2 6.1 5.6 6.0 11.0 9.0 17.0 12.0

As shown by Table 2 Bath I, prepared with ordinary elusive, are grouped in pairs and demonstrate the utility tap water and containing no additive, etched at a rate of succinic, azelaic, sebacic, and pimelic acids to prowhich was initially slow and further slowed fairly markedvide high capacity acidified peroxides etchants when 1y even at the low dissolved copper concentrations. )Bath combined with silver ions alone or in further combination I also had low capacity and exhibited an undesirably with phenacetin.

high degree of peroxide decomposition. Baths J and K In the following examples the copper clad laminates containing adipic acid and phenacetin, respectively, in were cut into board specimens having dimensions of combination with silver nitrate show substantial im- 9 x 9 x ,5 inch. Each of these specimens was then spray provement over Bath 1 and a fast: initial etch rate as etched using a Model 600 Spray Etcher manufactured by well as high capacity. Bath L containing the combinathe Chemcut Division of Centre Circuits Company tion of adipic acid, phenacetin and silver nitrate sur- (U.S.A.). The reservoir of the spray etcher was charged prisingly had higher capacity than either Baths J or with about 3 gallons of etchant solution and the spray K, demonstrating the improvement obtained by use of etcher set to apply about 5 gallons per minute to each specimen. Etch time was determined with a stopwatch and etch rate calculated after weighing each specimen before and after treatment.

Examples 2528 Four peroxide solutions were prepared for testing by the above outlined spray etching procedure. Each of the solutions contained about 6% hydrogen peroxide and 13% sulfuric acid such that the mol ratio of hydrogen peroxide to sulfuric acid was about 1 to 0.75. The Example 25 solution was prepared with ordinary tapwater and contained at least about 5 parts per million total free chloride and bromide ion. The Example 26 solution also contained no additive but was prepared using deionized water such that it contained only about 0.2 part per million of chloride and bromide ion. The Example 27 solution was prepared from the same tap water used in Example 25 and contained about 300 parts per million of phenacetin and 200 parts per million silver nitrate. The Example 28 solution was prepared from the same tap Water and 96 parts per million phenacetin, 180 parts per million adipic acid and 200 parts per million silver nitrate. All solutions were regulated during spray etching at a temperature of about 60 C. Results summarizing Examples 25-28 are given in Table 4.

12 effective not only in the presence of copper but also other metal ions. Thus, the acid solutions containing the phenacetin and sulfathiazole may be employed in the dissolution of other metals such as iron, nickel, cadmium,

5 zinc, germanium, lead, steel, aluminum and alloys containing a major portion of such metals. Aluminum metal is more effectively dissolved when the acid employed is nitric acid or fluoboric acid, particularly fiuoboric acid. The solutions are, however, less eifective on certain other metals such as gold, tin, chromium, stainless steel and titanium.

Although certain preferred embodiments of the invention have been disclosed for purpose of illustration, it will be evident that various changes and modifications may be made therein without departing from the scope and spirit of the invention.

We claim:

1. The method for dissolution of copper and copper alloys which comprises contacting the metal with an acidified aqueous solution containing 212% by weight hydrogen peroxide, about 045-5 .5 grams per liter hydrogen ion; and having incorporated therein a catalytic amount of an additive containing (A) a member selected from the group consisting of phenacetin, sulfathiazole,

silver ions, and mixtures thereof, and (B) at least parts As shown by Table 4, the peroxide solution of Example 4 25 containing no additive and prepared with ordinary tap water is unsuitable for practical use in spray etching with a high initial etch rate of 11 minutes and capacity less than 6 ounces of dissolved copper. Example 26 indicates improvement over the solutions of Example 25 i obtained in spray etching simply by reducing the total free chloride and bromide ion content below the 2 parts per million level. In Example 25 difficulty was encountered in pumping the solution through the spray etcher. Example 27 solution demonstrates that the addition of phenacetin and silver nitrate produces an etchant having high capacity and fast etch rates despite the use of tap water in makeup of the etchant. The etchant solution of Example 28 shows surprisingly that a substantial improvement in spray etching is obtained by substituting a combination of adipic acid and phenacetin in the same total amounts or less for the phenacetin in the etchant of Example 27.

The present invention is eminently suited for etching of copper in a highly efiicient and practical manner and substantially reduces the cost of etching in the manufacture of printed circuit boards as heretofore carried out with ammonium persulfate. In addition to etching of copper the invention may be applied generally in other conventional chemical dissolving operations such as chemical milling, graining and bright dipping or polishing. In such applications the temperature of the acid-peroxide solution may be varied, if desired, outside of the range prescribed for the etching of copper. For example, bright dipping operations may be carried effectively at room temperature or slightly above. The addition of phenacetin or sulfathiazole to acid peroxide solutions. is beneficially per million of a member selected from the group consisting of saturated dibasic acids of 4 to 12 carbon atoms and hydroxy and carboxyl substituted saturated dibasic acids of 4 to 12 carbon atoms and mixtures thereof, provided at least one carbon atom adjacent a terminal dibasic acid carboxyl group is free of said hydroxy and carboxyl substituents, said solution being regulated at a temperature between about 40-65 C.

2. The method of claim 1 in which the solution has incorporated therein between about 75 to 500 parts per million phenacetin, 75 to 500 parts per million dibasic acid and has a total free chloride and bromide ion content less than 2 parts per million.

3. The method of claim 1 in which the solution has incorporated therein between about 75 to 500 parts per million dibasic acid and between about 75 to 500 parts per million sulfathiazole.

4. The method of claim 1 in which the solution has incorporated therein between about 75-5 00' parts per million adipic acid and between about 50-500 parts per million silver ions.

5. The method for dissolution of copper and copper alloys which comprises contacting the metal with an acidified aqueous solution containing 212% by weight hydrogen peroxide, about 0.455.5 grams per liter hydrogen ion; and having incorporated therein (A) 75-500 parts per million of an additive selected from the group consisting of phenacetin and sulfathiazole and mixtures thereof; (B) at least about 10 parts per million free silver ions; and (C) at least 40 parts per million of a dibasic acid selected from the group consisting of saturated dibasic acids of 4 to 12. carbon atoms and hydroxy and 13 carboxyl substituted saturated dibasic acids of 4 to 12 carbon atoms and mixtures thereof, provided at least one carbon atom adjacent the terminal dibasic acid carboxyl group is free of said hydroxy and carboxyl substituents, said solution being regulated at a temperature between about 40-65 C.

6. A composition for metal dissolution comprising acidified aqueous hydrogen peroxide containing 2-12% by weight hydrogen peroxide, about 0.45-5.5 grams per liter hydrogen ion and having incorporated therein a catalytic amount of additive comprising the combination of (A) at least 30 parts per million of a member selected from the group consisting of phenacetin, sulfathiazole, silver ions, and mixtures thereof, and -(B) at least 40 parts per million of a member selected from the group consisting of saturated dibasic acids of 4 to 12 carbon atoms and hydroxy and carboxy substituted saturated dibasic acids of 4 to 12 carbon atoms and mixtures thereof, provided at least one carbon atom adjacent a terminal dibasic acid carboxyl group is free of said hydroxy and carboxyl substituents.

7. The composition of claim 6 having incorporated therein as additive between about 75 to 500 parts per million phenacetin and 75 to 500 parts per million dibasic acid and having a total free chloride and bromide ion content less than 2 parts per million.

8. The composition of claim 6 having incorporated therein as additive between about 75 to 500 parts per million dibasic acid and between about 75 to 500 parts per million sulfathiazole.

9. The composition of claim 6 having incorporated therein as additive between about 75-500 parts per million adipic acid and between about 50-500 parts per million silver ions.

10. The composition of claim 6 containing 223% by weight sulfuric acid.

11. A composition suitable for conversion to an acidified-hydrogen peroxide etchant containing 212% by Weight hydrogen peroxide and a catalytic amount of additive including phenacetin and at least parts per million dibasic acid, said composition comprising a concentrated aqueous solution containing 2070% hydrogen peroxide, between about 2004000 parts per million phenacetin, and between about 200 to 10,000- parts per million of a member selected from the group consisting of saturated dibasic acids of 4 to 12 carbon atoms and hydroxy and carboxyl substituted saturated dibasic acids of 4 to 12 carbon atoms and mixtures thereof, provided at least one carbon atom adjacent a terminal dibasic acid carboxyl group is free of said hydroxy and carboxyl substituents.

References Cited UNITED STATES PATENTS JACOB H. STEINBERG, Primary Examiner. 

1. THE METHOD FOR DISSOLUTION OF COPPER AND COPPER ALLOYS WHICH COMPRISES CONTACTING THE METAL WITH AN ACIDIFIED SOLUTION CONTAINING 2-12% BY WEIGHT HYDROGEN PEROXIDE, ABOUT 0.45-5.5 GRAMS PER LITER HYDROGEN ION; AND HAVING INCORPORATED THEREIN A CATALYTIC AMOUNT OF AN ADDITIVE CONTAINING (A) A MEMBER SELECTED FROM THE GROUP CONSISTING OF PHENACETIN, SULFATHIAZOLE, SILVER IONS, AND MIXTURES THEREOF, AND (B) AT LEAST 40 PARTS PER MILLION OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF SATURATED DIBASIC ACIDS OF 4 TO 12 CARBON ATOMS AND HYDROXY AND CARBOXYL SUBSTITUTED SATURATED DIBASIC ACIDS OF 4 TO 12 CARBON ATOMS AND MIXTURES THEREOF, PROVIDED AT LEAST ONE CARBON ATOM ADJACENT A TERMINAL DIBASIC ACID CARBOXYL GROUP IS FREE OF SAID HYDROXY AND CARBOXYL SUBSTITUENTS, SAID SOLUTION BEING REGULATED AT A TEMPERATURE BETWEEN ABOUT 40-65*C. 